Examples of the types of intravascular ultrasonic catheter arrangements to which the present invention may be applied are disclosed in our earlier United Kingdom Patent Nos. 2,221,267; 2,233,094 and our U.S. Pat. Nos. 5,081,993; 5,257,629 and 5,456,259.
With the kinds of ultrasonic transducer array to which the present invention relates, its very small size (typically one millimetre in diameter) means that there are considerable technological problems to overcome in order to firstly make it possible to manufacture the array at an acceptable yield level and secondly to provide the array and its associated control circuitry/software with an acceptable performance particularly as far as the definition of images obtained is concerned.
Ultrasound arrays to which the present invention is applicable have typically employed piezoelectric materials such as modified PZT (lead zirconate titanate) for the transduction of a radio frequency (rf) electrical signal into an ultrasonic signal. In very high frequency applications, such as are relevant to the present invention, the performance of such piezoelectric arrays is limited by the grain size of the piezoelectric ceramic material. This is because, due to the very small size of the array, the grain size of the material begins to become comparable with the dimensions of the array elements.
Furthermore, because of the crystalline nature of the material from which the elements of the array are manufactured, the manufacturing operations such as lapping, polishing, dicing and electroding (i.e. securing electrodes to the elements of the array) introduce defects which are associated both with the bulk body of the transducer element material and its surface. In particular microcracks are generated which significantly reduce the macroscopic fracture resistance of the material.